Heretofore, in production lines or the like in factories, conventional PLCs (Programmable Logic Controller) have been in use to input signals (ON or OFF information) relating to the conductive states from various input devices (switches, sensors and so on) and to control (to output ON or OFF signals or so to) output devices (relays, motors and the like) by the use of sequence programs in which the various input devices are combined in desired configurations (combined in serial or parallel).
When preparing a sequence program in which input devices arbitrarily selected from among various input devices are arbitrarily combined serially and parallelly, a user usually prepares a ladder circuit by the use of preparation supporting software for sequence programs, prepares data based on the prepared ladder circuit and inputs the date into a conventional PLC for enabling the conventional PLC to perform a desired operation.
In recent years, requirements are increasing for so-called safety PLCs which are specialized for safety over the conventional PLCs. In the safety PLCs, it is ideal that input devices ensured to be safe are used to make outputs to output devices those ensured to be safe. However, it may actually be the case that all the input devices are not those input devices ensured to be safe. Where a sequence program is prepared with the mixture of input devices ensured to be safe and those input devices not ensured to be safe, the user has to judge whether an output to an output device has been ensured to be safe or not.
In Patent Document 1 as prior art, there is proposed a safety controller which is capable of realizing fail-safe functions while being simplified in the construction of a unit alone and is capable of reducing the cost not only as the unit alone but also as an entire system.
Patent Document 1 is Japanese unexamined, published patent application No. 2002-358106.
When preparing a sequence program (ladder circuit or the like) by the use of preparation supporting software for sequence programs, it is possible for the user not only to use input devices of any kinds but also to make any combinations (serial or parallel combinations).
Therefore, it may occur that a sequence program can be prepared wherein input devices ensured to be safe and input devices not ensured to be safe are mixed and are complicatedly combined in serial or parallel. The judgment of whether or not the output section of the sequence program (in particular, the output section of a sequence program with several tens to several hundreds input devices being combined) prepared in this way has been carried out by the handwork of the user and has required massive manpower. Further, the judgment by the handwork of the user has involved a risk of wrong judgments.
The reason is that where input devices ensured to be safe are mixed with those not ensured to be safe, there exist a combination ensured to be safe (serial and the like) and another combination not ensured to be safe (parallel and the like).
In the prior art of Patent Document 1, although the reliability in arithmetic results is enhanced by placing a prepared sequence program under arithmetic processing by a plurality of CPUs to compare the results, it is unable to enhance the safeness of the sequence program itself.
Further, as other prior art, there is one in which settings have been made in advance regarding whether each input device is safe or not and in which any input device not ensured to be safe is distinguishably displayed when a sequence program (ladder circuit or the like) is displayed by the use of preparation supporting software for sequence programs. However, the display is made only of whether each input device itself is safe or not, and whether an output of the sequence program is that output ensured to be safe or not has to be judged finally by the handwork of user at the cost of massive manpower.
The present invention has been created taking the aforementioned respects into consideration, and the problem to be solved is to provide a safety PLC, preparation supporting software for sequence programs and a judgment method for sequence programs which are capable of automatically judging whether an output section of a sequence program is the output ensured to be safe or not and capable of enabling a user to judge whether the output section is the output ensured to be safe, within a short period of time and precisely.